Prevent building static version of python library

http://bugs.gentoo.org/show_bug.cgi?id=298881

--- python/Makefile.am
+++ python/Makefile.am
@@ -8,7 +8,7 @@
 
 pythonlibdir            = $(libdir)/@PYTHON_VERSION@/site-packages
 pythonlib_LTLIBRARIES   = _libpst.la
-_libpst_la_LDFLAGS      = -module $(NO_UNDEFINED) -avoid-version
+_libpst_la_LDFLAGS      = -module $(NO_UNDEFINED) -avoid-version -shared
 _libpst_la_SOURCES      = python-libpst.cpp
 _libpst_la_LIBADD       = -lboost_python ../src/libpst.la